Tool Briefing: Shocking ignition diagnostics

Figure 3B: The ECOP from ATS allows a technician to get an ignition waveform on a labscope (here the ATS EScope) without having to back probe any wires. For more information on both these products check out www.VehicleServicePros.com/10889178 and www.VehicleServicePros.com/10106854.

Photo credit: ATS

Figure 1: Scan tools, like the Autel MaxiDAS DS708, can quickly tell us which cylinder has elevated misfires by scanning for misfire DTCs and looking at the misfire counter in Mode 6, as seen here. For more information on the MaxiDAS DS708 check out www.VehicleServicePros.com/10176527.

Photo credit: Autel

Figure 2: Here, a wiring diagram from Mitchell1's ProDemand quickly shows us that the gray/black wire is the ignition coil ground. Knowing this, you can backprobe the wire in order to get an ignition signal. For more information on this product check out www.VehicleServicePros.com/10451541.

Photo credit: Craig Truglia

Figure 3A: Here, a technician is hooked up with a labscope to an ignition coil, by back probing its ground with an OTC Tech Scope, No. 3857. For more information on this product check out www.VehicleServicePros.com/10615029.

Photo credit: OTC

Figure 3B: The ECOP from ATS allows a technician to get an ignition waveform on a labscope, here the ATS EScope, without having to back probe any wires. For more information on both these products check out www.VehicleServicePros.com/10889178 and www.VehicleServicePros.com/10106854.

Photo credit: ATS

Figure 4: The waveform given by a 2006 Jeep Liberty's coil is the "prototypical" waveform. Most ignition waveforms feature a period where the coil is powered up when the PCM is grounding it and then a sudden spike upwards when the PCM's driver opens the circuit and the energy shoots out of the ignition coil through the spark plug. Interpreting this waveform for nuances can reveal a great deal about a driveability problem.

Photo credit: Craig Truglia

Figure 5: A bad ignition coil on a Ford F150.

Photo credit: Craig Truglia

Figure 6: A good ignition coil on a Ford F150. Take note that the coil oscillations are more even.

Photo credit: Craig Truglia

Figure 7: 2004 Kia Sorrentos only have three coils. Here, the bad one has a much higher voltage, due to it drawing much too low an amperage to fire correctly.

Photo credit: Craig Truglia

Figure 8: With the bad ignition coil replaced, the voltages are now fairly even. It can be seen that one coil is still a little weaker than the others, but the vehicle did not misfire so this was merely noted by the shop just in case there was an issue in the future.

Photo credit: Craig Truglia

Figure 9: Using GTC's TA500 SmarTach+COP Multisystem Ignition Analyzer, a good ignition coil will give readings consistent with the other ignition coils on the vehicle. For more information on this tool check out www.VehicleServicePros.com/10237134.

Photo credit: Craig Truglia

Figure 10: A bad ignition coil on most vehicles will show a vastly different reading than the good ones. Here, on this 2006 Scion xB, the bad ignition coil fails to fire altogether. After swapping the coils and taking readings again, the shop confirmed that the coil alone was defective.

Photo credit: Craig Truglia

Figure 11: The compression between the two cylinders in the 2006 Scion xB was virtually identical, showing the engine was in sound mechanical condition. For more information on the Snap-on Compression Tester Gauge Set, No. MT308L, check out www.VehicleServicePros.com/10984357.

Photo credit: Craig Truglia

Common ignition waveforms have a high firing voltage and a measurable dwell period, as seen here on the Picoscope. For more information on this tool check out VehicleServicePros.com/11063100.

Photo credit: Picoschope

CUSTOMER CONCERN:

Vehicle misfires

VEHICLE APPLICATIONS:

2006 Scion xB

Tests/Procedures for C.O.P. ignition diagnostics:

Use a scan tool to check which cylinder is setting the DTC.

If possible, use misfire counter on Mode 6 or in the OEM enhanced section of the scan tool to count misfires per cylinder.

Test the suspect ignition coil using a labscope or C.O.P. tester.

Swap coils and see if the misfire changes cylinders.

TOOLS USED:

Scan tool

Labscope

Compression gauge

Wiring diagrams

C.O.P. tester

Fuel injectors do not go bad like they used to and engines routinely last 200,000 miles when maintained properly. Usually, when a misfire occurs on a vehicle these days, the easy assumption to make is to replace the coil in the cylinder setting the misfire DTC.

However, the PCM might misinterpret the CKP sensor and set a misfire DTC for the wrong cylinder. Furthermore, bad engine compression and irregular injectory spray patters can affect ignition in the cylinder.

Thankfully, with the right tools and techniques, ignition misfires can be diagnosed with almost 100 percent certainty rather quickly.

2006 Scion xB bad ignition coil

Unlike Fords, Toyotas are not quite so notorious for bad ignition coils. So, when one of these vehicles rolls into the shop with a misfire, you probably assume the vehicle needs spark plugs. This is because customers presume their Scions are like most other vehicles and only need spark plugs once every 100,000 miles, as opposed to 30,000 miles such as their factory maintenance schedules call for.

What do you do if after the "tune up" the misfire remains? The following steps cover how to quickly discern the cause of the misfire.

1. Check which cylinder is misfiring

Using a scan tool, check the misfire DTC. Depending on the scan tool (and vehicle) you might be able to count misfires using Mode 6, as seen in Fig. 1. On some vehicles, a relative compression, cylinder power balance and fuel injector flow test can be done with a click of a button. These tests help the technician quickly find out which cylinder is misfiring and depending upon the bidirectional controls offered by the scan tool, narrow down if the misfire is caused by an ignition, fuel or mechanical issue.

2. Test the suspect ignition coil

Testing ignition coils is an uncomplicated process, though ignition waveform interpretation can be quite complicated. For reasons of space, we are not going to discuss every nuance of interpretting an ignition waveform. Instead, we will cover both the old school and new school in ignition diagnostics.

Old school. You will need a labscope, wiring diagram and a T-pin to diagnose ignition the old school way. With the vehicle off, use a wiring diagram to find out which wire on the harness is the ground to the ignition coil, such as that in Fig. 2. Then, simply slide the T-pin into where the ground wire merges into the connector. (See Figs. 3A and 3B.)

It should be noted that if you do not have access to an accurate wiring diagram, simply keep putting the T-pin into a different wire until you get an ignition signal with the vehicle running. You will get a waveform on both the ground and power sides of the ignition circuit, so it is a matter of preference which side you choose.

Do this to each coil using different leads so you can compare ignition waveforms from different ignition coils. The more channels your labscope has, the better.

As Figs. 4 to 6 show, an ignition waveform looks different on different OEM applications. However, you generally do not need to memorize a known good waveform. A bad coil will simply stick out like a sore thumb, looking different than the other ignition coils. Even in Figs. 7 to 8, where not a single coil looks alike, the one that was misfiring has much more voltage and it matches up with the msifire DTC on the scan tool.

It is important to note that when looking at an ignition waveform, the large spike in the beginning of the waveform is the firing voltage. On a current waveform, the waveform will be at its lowest point, because as voltage increases amperage falls.

The long line of voltage follwing the firing voltage is the firing time, which is the time elapsed when the spark is actually occurring. Differences in firing voltage and other parts of the waveform reflect conditions in the cylinder.

New school. Don't feel like looking up wiring diagrams or hooking stuff up? Simply use a GTC SmarTach+ COP tester. (See Figs. 9 and 10.) By placing its probe on the top of an ignition coil, it instantly gives an engine RPM, burn time and coil kV reading. So, instead of comparing waveforms, you simply comprare burn time and coil kV between coils.

This method can also be used to check misfirng cylinders on vehicles that use spark plug wires. The probe can be pointed at different spark plug wires. All the technician needs to do is compare results.

3. Swap coils and test again.

It is important to note that regardless of whether you are diagnosing ignition the old or new school way, ignition is always affected by conditions in a cylinder. So, low cylinder compression from a washed down cylinder can make a good coil look bad. (See Fig. 11.)

Simply swap a known "good" coil with the suspect one, and check again your readings. If the misfire changes cylinders, simply replace the suspect ignition coil. If it doesn't, the vehicle has a fuel or mechanical problem in that cylinder.

Presuming the reading does not change, the next step would be to remove the spark plug and do a compression test. If the vehicle does not have a dead misfire, it might be wise to check timing with a pressure transducer or do a cylinder leak down test. Now, if everything checks out, it is time to start testing the fuel injectors.

As for this 2006 Scion xB, the misfire changed cylinders, so it was fixed with a new coil.